It has long been known that when cost is no object, and the ultimate in electromagnetic shielding of air vents is necessary, hexagonal metal honeycombs which are held together by soldering or welding are the material of choice. Because all abutting surfaces of the undulations of the hexagon that form the honeycomb pattern are held together by a conductive bond, the maximum electromagnetic shielding which can be derived from a body of honeycomb material is in fact attained.
Such material is very costly. Its cost can be justified for very critical installations, especially for military applications where cost is subordinate to reliability and capacity. This is not the situation for most commercial (and also for many military) applications. For example, while an ultimate shielding of about 120 dB at relatively high frequencies (1 GHz) can be attained with soldered or welded-together honeycomb structure, there are many applications where only about 40 to 60 dB is necessary, and in which the cost of soldered or welded honeycomb for the panels cannot be justified.
This is not a new problem, and previous efforts have been made to utilize honeycomb which is constructed of undulations of metal foil which are joined at abutting segments by an epoxy resin. Such foils, without further treatment, do provide some shielding, perhaps as high as 60-70 dB (at 1 GHz) but also as little as 20 dB. This lack of reliability renders the use of such honeycomb filters unuseful for many important applications.
Because the shielding property is increased by conductively joining the undulations together, numerous means conductively to join them have been suggested. One way is to pierce the abutting segments of the undulations with a probe (pin hole) that carries some of the metal from one undulation to its neighbor. This does provide some interconnection between the undulations, but its effect is rather small, and is not uniform, predictable or reliable.
Another means is by electroless nickel-plating the entire honeycomb structure. This will provide, while using an epoxy assembled honeycomb, performance nearly equal to that of a soldered or welded honeycomb. The problem is the cost. The surface area of the honeycomb is large, and therefore so is the cost of nickel-plating it. Due to the cost one is as well off using the soldered or welded honeycomb.
Yet another suggested arrangement is to electroplate the honeycomb structure with a coating such as tin or cadmium. Again the cost is relatively high, although not as high as the cost of a honeycomb whose entire surface has been nickel plated or having the undulations soldered or welded together. This arrangementhas been known to add additional shielding to the honeycomb panel. However, it can also suffer from the acid treatment. If the contact points created by the sawing operation in creating the panels from the honeycomb blocks are etched off, a reduction in the shielding quality of the panel can be significantly reduced (a loss of as much as 40 dB is not unusual).
It is an object of this invention to provide a cost effective honeycomb panel by performing only mechanical processes on the ends of the panel.
Honeycomb panel wafers are formed from slices cut from large blocks of honeycomb by a sawing operation. The teeth of the saw tend to sever the panel, and while they do, some of them pull shard-like portions of one undulation across the epoxy gap, where they touch against the opposite undulation. This provides a few conductive bridges between the abutting undulations. In practice the undulations are aluminum alloy about 0.002 inches thick, and the epoxy bond is about 0.0005 inches thick. However, the presence location and number of these bridges is random and not reliable. Their formation is subject to variations of the sharpness and accuracy of the teeth, the speed of the saw cut and the pressure of the saw against the honeycomb. With so many variables, repetitive consistency is not be anticipated. Still, some improvement in shielding performance is noticeable compared to panel which does not have these bridges.
It is an object of this invention to perform a mechanical finishing operation on the ends of a panel to provide a substantially continuous and reliable metal bridge from one undulation to its neighbor, made of self-material from one of the undulations, thereby "blending" the undulations together at their ends to make a continuous and reliable metallic bridge between them, all of this at a modest cost.
The process reliably produces in a panel, with 1/8 inch cells of about 1/4 inches thick, shielding on the order of 70-80 dB at (1 GHz).